1. Field of the Invention
The present invention is in the field of light modulation devices particularly for infrared radiation wherein thin films of vanadium dioxide are caused to change their optical and electrical characteristics such as reflectance and conductivity in response to thermal stimuli. More specifically, the present invention relates to a novel device structure that is operable as a high-speed infrared shutter or modulator.
2. The Prior Art
The use of reversible thermodynamic transitions between the semiconductor and metal states for vanadium dioxide crystal as a means of thermally modulating its reflectivity, thereby achieving the function of an optical modulator, appears to have been first disclosed in U.S. Pat. No. 3,484,722 by Barker, et al. As the temperature of a vanadium dioxide crystal is raised, the crystal changes from a slightly reflective semiconductor below about 68 degrees C. to a highly refective metallic phase at temperatures above about 68 degrees C.
Although the transition effect can easily be demonstrated under laboratory conditions by a human operator who gradually increases or decreases the temperature of the vanadium dioxide film, it is much more difficult to devise a system which will produce hundreds or thousands of transitions each second without human intervention. The present invention addresses this latter problem.
In U.S. Pat. No. 4,283,113 Eden disclosed an apparatus for switching light between optical fibers. The apparatus comprises a vanadium dioxide thin film switchable between high reflectence (above 68 degrees C.) and high transmission (below 68 degrees C.) states. Eden positions a series of thermoelectric junctions around the edge of the film and in thermal contact with the film for selectively heating and cooling the film above or below its transition temperature to switch the film between its transmitting and reflecting states. Switching rates of several hertz are mentioned.
Other inventors, notably Mattis in U.S. Pat. No. 3,664,726 and McConnell, et al. in U.S. Pat. No. 3,834,793 disclose devices in which thermochromic materials are used for modulating light.
The use of thermoelectric (TE) coolers for selectively heating and cooling the vanadium dioxide thin film and substrate above or below its transition temperature (68 degrees C.), as suggested in U.S. Pat. No. 4,283,113 to Eden, can achieve at best a switching speed of a few hertz, even with the latest available TE cooler technology. The use of resistive type heaters for heating the vanadium dioxide film and substrate above the transition temperature as disclosed in U.S. Pat. No. 3,664,726 to Mattis does not switch any faster. In this case, one has to take into consideration also the cooling time needed to return the film and substrate to below the transition temperature.
Optical modulators that are capable of switching at only a few hertz have very limited applications in communication and instrumentation areas. Minimum switching speeds of at least tens of hertz to hundreds of hertz are ordinarily required in most applications.